In my previous post I said that Plesiosaurs would have had to adapt to the freshwater conditions of Patagonian lakes, which is not a simple feat for an animal that lived in the salty sea waters.
Salt water – Freshwater adaptation
In all aquatic creatures, the outer or external environment is separated from the creature’s cells by a membrane through which different substances move.
These membranes are of two kinds, one, at a cellular level, the other at an organism’s level (its skin).
Solutes (salts, minerals, etc.) are diluted in the solvent (water) in different concentrations. If a membrane separates them, the solutes will migrate across it from the area with higher concentration to the area with lower concentration.
So, salt water animals whose bodies have less salt diluted in their organisms in comparison to the sea water in which they live, must avoid dehydration (due to intake of salt from the environment). They tend to lose water and gain salt.
Freshwater fish on the other hand must avoid loss of salts (their blood and cells have a higher concentration than the surrounding freshwater).
Adapting to a freshwater environment
This is known as osmoregulation. If a marine animal moves into a freswater habitat, its regulating system would have to be reversed, a virtually impossible feat.
Look at sharks. They remove excess salt from their blood through their kidneys. They uptake water from the sea and eliminate the salt via urine. If they move into freshwater, they will absorb too much water and die.
Marine vertebrates evolved on land and their blood has a lower concentration than seawater. As these animals breathe air from the atmosphere their respiratory surfaces (unlike fish, whose gills are submerged in water) are not in contact with seawater. This, plus their “waterproof” skins, reduces the surface through which water loss can occur.
Yet they still lose water when excreting uric acid (reptiles do this, mammals excrete urea), and while breathing. They absorb salts through food or by drinking sea water. So they must solve the problem of losing water and up-taking too much salts.
Marine reptiles such as turtles, crocodiles, sea snakes, iguanas (and probably plesiosaurs) must drink seawater to keep alive. However they can not produce concentrated urine like fish do. To compensate, they must secrete salt and are equipped with special glands that do just that; they pump out the Chlorine ions from the animal’s bodies (the sodium ions follow suit).
Reptiles have difficulties in osmoregulating in freshwater because their skin is relatively impermeable it hampers the influx of water and the excretion of salts across it. Therefore they must produce dilute urine and reabsorb the salts in its urinary tract.
However, it seems that there were freshwater plesiosaurs after all as some remains have been found in Australia, Canada and South Africa  in non-marine strata.
The remains are mostly juvenile specimens, which suggests that these animals may have entered river mouths or coastal lagoons to escape from their predators. So their environment may have been marine and some died in a brackish/non-marine depositional environment.
Among these freshwater plesiosaurs is the Leptocleidus ( Greek for "thin clavicle"). This is an Early Cretaceous animal and is similar to more archaic reptiles. Some authors believe that it retained primitive features and that this was " a consequence of the freshwater, probably fluviatile, habitat of this Plesiosaur, which resulted in its leading a life sheltered from the great competition among the marine Plesiosaurs". Leptocleidus was similar to a modern seal in size.
So, there is a very faint chance that some plesiosaur could have lived in freshwater and having survived until now, live in modern lakes. (Faint and I would like to add, virtually zero probability).
 Kear, Benjamin (2006). Plesiosaur remains from Cretaceous high-latitude non-marine deposits in Southeastern Australia. Journal of Vertebrate Paleontology 26, 196–199
 Vandermark, D., J. A. Tarduno, and D. B. Brinkman. (2006). Late Cretaceous Plesiosaur Teeth from Axel Heiberg Island, Nunavut, Canada. Arctic. 59, no. 1: 79-82.
 Cruikshank, A. R. I. (1997). A lower Cretaceous Pliosauroid from South Africa. Annals of the South African Museum 105: 206-226.
 Andrews, C,W,. (1922). Description of a new plesiosaur from the Weald Clay of Berwick (Sussex). Quarterly Journal of tlte Geological Society of London 78: 285-298.
Patagonian Monsters - Cryptozoology, Myths & legends in Patagonia2010 International Year of Biodiversity Copyright 2009-2010 by Austin Whittall ©